1. Field of the Invention
The present invention relates to a protective relay system that is capable of protecting a power system by reliably eliminating a fault point when a fault occurs while the power system is oscillating.
2. Description of the Related Art
A prior art protective relay system will now be described with reference to FIG. 17.
FIG. 17 is a diagram showing a relationship between an existence region (a load region) of distance-measuring impedance Z caused by a power flow under sound load conditions of a power system and a fault direction detecting element (mho characteristics and ohm characteristics). Under the sound load conditions, the existence region of distance-measuring impedance Z is located away from the operating range of the fault direction detecting element. Thus, neither a mho relay nor a blinder relay operates to detect a fault unnecessarily.
If, however, the power system increases in power flow, the distance-measuring impedance approaches the operating range of the fault direction detecting element. It is thus likely that when the power system oscillates, the distance-measuring impedance will fall within the operating range of the fault direction detecting element and the relays will operate unnecessarily to trip a circuit breaker though no faults occur.
While the power system is oscillating, the relay system usually detects the oscillation and locks an operation signal output from a distance relay including the fault direction detecting element.
A method for detecting a system oscillation is described in Electric Technology Research, Vol. 37, No. 1, p. 65. According to this Research, the oscillation is detected by considering a change in the locus of impedance to be a difference in operation time between a mho relay and an offset mho relay as direction detecting elements having different operating zones or a difference in operation time between two blinder relays.
FIG. 18 shows impedance characteristics appearing when a system oscillation is detected by a difference in operation time between a mho relay 11-2 and an offset mho relay 11-1. When the locus of impedance Z is moved by the system oscillation as indicated by arrow 2, the offset mho relay 11-1 operates at time t1 and the mho relay 11-2 operates at time t2. If a time difference (t2−t1) is not smaller than a set value, a system oscillation detection relay (PSB) 11 determines that a system oscillation (including a loss of synchronization) occurs and locks the operation of the distance relay. The lock is released after a fixed period of time elapses after the locus of impedance Z falls outside the operating range of the offset mho relay 11-1 in order to prevent a mistrip. FIG. 19 illustrates an example of the system oscillation detection relay (PSB) 11.
Referring to FIG. 19, the relay 11 includes an offset mho relay 11-1, a mho relay 11-2, a NOT circuit 11-3, and an AND circuit 11-4. The AND circuit 11-4 has an operating condition that is met when the offset mho relay 11-1 operates and the mho relay 11-2 does not operate. The relay also includes an on-delay timer (TDE) 11-5 that outputs a signal “1” when the signal “1” of the AND circuit 11-4 continues to be output for not shorter than time T20.
In other words, when time (t2−t1) required from when the impedance Z falls within the operating range of the offset mho relay 11-1 until it falls within the operating range of the mho relay 11-2 is longer than setting time T20 of the on-delay timer (TDE) 11-5, the system oscillation detection relay (PSB) 11 determines that the power system is oscillating and the on-delay timer 11-5 outputs a system oscillation detection signal “1.”
When the on-delay timer 11-5 detects the system oscillation, an off-delay timer 11-6 continues to output the system oscillation detection signal “1” for a time period of T21.
In FIG. 19, a PSB output means a system oscillation detection signal.
The system oscillation detecting relay 11 is basically configured such that the relay 11 locks an operation signal output from the distance relay whenever it detects a system oscillation and then continues the lock (holds the preceding value) even when a fault occurs in a protective area for a power transmission line. This means that the prior art protective relay system does not operate erroneously when a fault occurs in a protective area during the system oscillation.
The system oscillation is a phenomenon appearing in three-phase equilibrium. When a fault occurs during the system oscillation, the lock of an operation signal output from the protective relay system is released in accordance with the level of a zero-phase-sequence current, a negative-phase-sequence current, or the like. However, this method does not take into consideration the selectivity of a fault direction (Electric Technology Research, Vol. 37, No. 1, p. 66).
It is thus likely that the protective relay system will operate unnecessarily due to zero-phase-sequence and negative-phase-sequence currents caused by a fault occurring outside a protective area and by unbalanced components in an open-phase state of a single-phase reclosing relay.
A direction determination element of a direction comparison distance relay, which uses a double polarity voltage using an amount of electricity of a fault phase and that of electricity that is unsusceptible to variations in phase before and after a fault as a reference value, is in practical use (disclosed in Jpn. Pat. KOKOKU Pub. No. 64-6608).
On the other hand, a user has recently desired to quickly eliminate a fault that has occurred in a protected area during the oscillation of a system. However, there is a problem that a fault is difficult to determine by a direction determination element using the above polarity amount during the oscillation. The reason is as follows. The oscillation varies a system voltage, an amount of current, and a phase, and an amount of polarity necessary for determining a fault direction is not fixed, with the result that the fault direction cannot be determined correctly.
As the most remarkable examples of system oscillation, FIGS. 20A to 20C show vectors of voltage and current during a loss of synchronization. The diagrams of vectors are shown using a voltage of phase A as a reference voltage. It is seen from FIGS. 20A to 20C that the vectors of current vary from moment to moment. In FIG. 20A, a relationship between voltage and current is almost close to the state of a receiving power flow. In FIG. 20B, the relationship is close to the state in which a fault occurs. In FIG. 20C, the relationship is close to the state of a sending power flow. Such an amount of electricity accompanied with variations in phase is not suitable for an amount of polarity.